Humans build a secret army of immune system defenders that quickly launch a call to arms against viruses, bacteria and other pathogens that they've never seen before, a team of Stanford University School of Medicine researchers said in a paper published Thursday in the journal Immunity.

What's more, the researchers said, these defenders are multi-taskers that can acquire memory to thwart a specific invader as well as various unrelated pathogens.

The findings throw into question a long-held scientific belief that CD4 cells, which unleash the immune system against viruses like HIV, develop pathogen-specific memory only by coming in contact with specific invaders or by being instructed to defend against a pathogen through a vaccine.

Instead, said Mark Davis, lead author of the paper, the research suggests that exposure to the specific microbe may also help the body defend against a wide array of other pathogens. Bottom line, Davis said, it may not be all that bad to let your child eat dirt.

I spoke with Davis, a professor of microbiology and immunology and director of the Institute for Immunity, Transplantation and Infection at Stanford, about his team's findings and what they mean for mud pie makers worldwide.

Q.: How do these CD4 cells develop this acquired memory of a virus, for example, without ever having been exposed to that virus?

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